In the prior art, power factor correction has been used in a number of environments because improved power factor results in decreased root mean square (RMS) alternating current (AC) line current drawn by the device having power factor correction. For example, the power factor of a standard (uncorrected) power supply is typically in the range of 0.5 to 0.65. Power factor correction can improve power factor to the range of 0.95 to 0.99. Such an improvement in power factor may decrease RMS AC line current consumption by up to 50%.
Power factor correction may be provided by either active components or passive components. Active power factor correction is typically used in power supplies rated at 200 Watts or more and is in the form of a power factor preregulator which is a complex device interposed between an input rectifier bridge and a bulk filter capacitor of a power supply. Use of active power factor correction provides input AC current signals that approximate a sine function with low levels of distortion; however, active components increase the cost of a power supply. Active power correction may increase the cost of a power supply by as much as 25-30%. Therefore, active power factor correction is typically used in high end power supplies.
Passive power correction is often used for power supplies rated at less than 200 Watts. Passive power correction circuits provide greater reliability at a lower cost than active power correction circuits for power supplies up to 200 Watts. In the prior art, at approximately 200 Watts, active power factor correction becomes less expensive because high-power passive elements such as capacitors and inductors become more expensive as power rating increases. Passive power factor correction is typically provided either by a low frequency inductive circuit, or by a capacitor-diode (C/D) circuit. Each type of circuit provides power factors in the range of 0.9 to 0.95. However, in the prior art these passive power correction circuits are typically not used for power supplies that provide over 200 Watts of power because costs of passive power correction equal or exceed costs of active power correction.
What is needed is a cost-effective passive circuit that provides power factor correction for power supplies that supply more than 200 Watts of power.